Advancing age, as well as injury, can lead to changes in the bones, discs, joints, and ligaments of the spine, producing pain from nerve root compression. Under certain circumstances, alleviation of pain can be provided by performing a spinal fusion. This is a procedure that involves joining two or more adjacent vertebrae with a bone fixation device so that they no longer are able to move relative to each other. For a number of known reasons, bone fixation devices are useful for promoting proper healing of injured or damaged vertebral bone segments caused by trauma, tumor growth, or degenerative disc disease. The external fixation devices immobilize the injured bone segments to ensure the proper growth of new osseous tissue between the damaged segments. These types of external bone fixation devices often include internal bracing and instrumentation to stabilize the spinal column to facilitate the efficient healing of the damaged area without deformity or instability, while minimizing any immobilization and post-operative care of the patient.
One such device is a bone fixation plate that is used to immobilize adjacent skeletal parts such as bones. Typically, the fixation plate is a rigid metal or polymeric plate positioned to span bones or bone segments that require immobilization with respect to one another. The plate includes a plurality of screw openings, e.g., either holes or slots, for receiving bone screws that are used to fasten the plate to the respective bones so that the plate remains in contact with the bones and fixes them in a desired position. Bone plates can be useful in providing the mechanical support necessary to keep vertebral bodies in proper position and to bridge a weakened or diseased area, such as when a disc, vertebral body, or fragment has been removed.
Implantation of a spinal fixation plate can be difficult, as each plate must be properly aligned with the vertebral bodies, and holes for receiving the bone screws must be drilled into the vertebrae at precise angles. It is often necessary to use the bone plate as a drill guide for drilling and tapping the bone in preparation for receiving the bone screws. Such a procedure can be difficult, however, as the surgeon is required to securely and rigidly hold the bone plate against the vertebrae, obtain proper alignment, drill, tap, and finally set the bone screws. Thus, drill guide devices have been developed that facilitate plate implantation. These devices typically include one or more lumens extending therethrough that are configured to align with the screw bores formed in the fixation plate. Some drill guides serve only as a guide for a drill bit, and other drill guides having larger diameters that allow a screw to pass through them. While the later is preferred, as removal of the guide is not necessary for screw placement and the guide facilitates alignment of the screw, drill guides having lumens with a larger diameter for receiving a fastening screw cannot center a drill bit since the drill bit must have a diameter that is smaller than the diameter of the fastening screw head. Some secondary instruments have been provided to center the drill bit within the drill guide lumen, however the use of additional devices is undesirable as it complicates the surgical procedure.
Accordingly, there remains a need for a drill guide device that includes a self-centering sleeve for centering a drill bit with respect to a drill guide lumen. It is also desirable to provide a self-centering drill guide device that includes a variable depth-stop mechanism.